TURBULENCE STRUCTURAL-CHANGES FOR A 3-DIMENSIONAL TURBULENT BOUNDARY-LAYER IN A 30-DEGREES BEND

A three-dimensional turbulent boundary layer (3DTBL) was generated on the floor of a low-speed wind tunnel by the imposition of a cross-stre am pressure gradient using a 30-degrees bend in the horizontal plane. The surface streamlines were deflected by as much as 22-degrees relati ve to the local tunnel centreline. Downstream of the bend, the 3DTBL g radually relaxed towards a 2DTBL; this was an impulse-and-recovery exp eriment which focused on the outer layer. Mean velocities were measure d with a three-hole yawmeter and turbulence quantities, which included the Reynolds-stress tensor and the triple products, were measured wit h a cross-wire hot-wire anemometer. The experiment isolated the effect s of crossflow from those of adverse streamwise pressure gradients, wh ich may have clouded interpretations of previous 3DTBL experiments. In particular, the detailed developments of the cross-stream shear stres s and of the stress/energy ratio become clearer. The shear-stress vect or lagged behind the velocity-gradient vector as crossflow developed; however, the two vectors became more closely aligned downstream of the bend. Reductions in the stress/energy ratio implied that crossflow ma de shear-stress production less efficient. Another effect of three-dim ensionality was a change of sign in the vertical transport of turbulen t kinetic energy by turbulence, in the inner part of the boundary laye r.